Pat the sushi-grade tuna dry with paper towels. Using a very sharp knife, trim away any sinew and dice the tuna into small, uniform 1/4-inch (6 mm) cubes. Place diced tuna in a chilled bowl to keep it cold while you assemble other components.

Season the tartare

In a small bowl whisk together 2 tablespoons extra-virgin olive oil, 1 tablespoon soy sauce, 1 tablespoon fresh lemon juice, the minced shallot, chopped capers, sliced chives, 1 teaspoon sea salt and 1/2 teaspoon freshly ground black pepper. Pour the dressing over the diced tuna and gently fold until evenly coated. Taste and adjust seasoning (add a touch more lemon or soy if needed). Keep chilled until plating.

Avocado Purée

Halve and pit the avocado. Scoop flesh into a bowl and mash with a fork until mostly smooth. Add 1 tablespoon fresh lime juice, 1 teaspoon extra-virgin olive oil and 1/2 teaspoon sea salt. If the purée is too thick, add up to 1 teaspoon cold water to loosen to a silky texture. Taste and adjust seasoning. For an ultra-smooth texture, press through a fine sieve or briefly pulse in a blender, then chill until plating.

Tomato Concassé

Bring a small pot of water to a boil. Score a small X on the bottom of each tomato and blanch in boiling water for 20–30 seconds until the skins begin to peel, then transfer to an ice bath. Peel, core and seed the tomatoes, then dice the flesh into small 1/4-inch pieces. Toss the diced tomato with 1 teaspoon extra-virgin olive oil, 1 teaspoon chiffonaded basil, 1/4 teaspoon sea salt and a pinch of freshly ground black pepper. Let sit briefly to meld flavors and drain any excess liquid before plating.

Bread Chips

Preheat oven to 375°F (190°C). Arrange baguette slices on a baking sheet. Brush each slice lightly with the 1 tablespoon extra-virgin olive oil and sprinkle with 1/4 teaspoon sea salt. Bake 8–12 minutes, turning once, until golden and crisp. Remove and let cool. Alternatively, toast in a skillet over medium heat until golden on both sides.

Assembly & Garnish

Forming the tartare

On chilled plates, spoon or pipe a neat bed of avocado purée (about 1–1 1/2 tablespoons per serving) where the tartare will sit. Using a ring mold (2.5–3 inches) place it on top of the purée and pack the dressed tuna tartare into the mold, pressing lightly to form a compact tower. Remove the ring carefully.

Spoon a small amount (about 1 tablespoon) of tomato concassé either on top of the tartare or to the side for contrast. Scatter a few chopped capers and a few microgreens around and on top. Arrange 3 bread chips per serving around the tartare. Finish with a small drizzle of extra-virgin olive oil and a light squeeze of lemon or lime over the tuna just before serving.

Serve immediately so the bread chips remain crisp and the tuna stays chilled.